Block copolymers have been developed rapidly within the recent past, the starting monomers usually being monoalkenyl arenes such as styrene or alphamethyl styrene block polymerized with conjugated dienes such as butadiene and isoprene. A typical block copolymer of this type is represented by the structure polystyrene-polybutadiene-polystyrene. When the monoalkenyl arene blocks comprise less than about 55% by weight of the block copolymer, the product is essentially elastomeric. Moreover, due to their peculiar set of physical properties they can be referred to more properly as thermoplastic elastomers. By this is meant polymers which in the melt state are processable in ordinary thermoplastic processing equipment but in the solid state behave like chemically vulcanized rubber without chemical vulcanization having been effected. Polymers of this type are highly useful in that the vulcanization step is eliminated and, contrary to scrap from vulcanized rubbers, the scrap from the processing of thermoplastic elastomers can be recycled for further use. Such block copolymers may also be hydrogenated to produce polymers having improved oxidative stability, along with other improved properties.
Triblock or multiblock copolymers such as S-B-S or S-EB-S have a balance of high tensile strength and low modulus that makes them excellent materials for applications such as adhesives, footwear and blending with other thermoplastics and thermosets. Properties such as creep, hysteresis and tensile strength improve as molecular weight increases for triblock copolymers. However, production is also increasingly difficult as molecular weight increases because solution viscosity increases markedly with molecular weight and time required for polymerization increases. As the time in the reactor increases, impurities and thermal termination cause more chains to be prematurely terminated. Therefore, it would be advantageous to make triblock copolymer and multiblock copolymers by coupling diblock molecules of one half the final molecular weight (or less in the case of stars) after the polymer has been finished in the plant. Furthermore, most of these polymers are sold as a component for blending. Therefore, a convenient place for coupling would be in the extruder during blending.
A new process has now been discovered that permits the production of such block copolymers in a more efficient manner.